Cthulhu Mythos is a generic name, coined by the writer August Derleth, for a particular type of horror story by the writer H. P. Lovecraft, or one of his followers, which deals with the fictional pantheon of the (so-called) Old Ones. These were immensely powerful beings that once ruled the Earth, but the universe changed (in some unspecified way, e.g., perhaps Planck's constant altered) such that they could no longer live in it. Many fled the old universe, awaiting another change in the universe, the day "when the stars are right" and they can return to claim what they believe to be their rightful inheritance. The high priest of the Old Ones, Cthulhu, remains on earth in the sunken city of R'lyeh, beneath the Pacific Ocean, in a death-like sleep, until the day when the stars are right again, and he can awaken. As Abdul Alhazred said, in the Necronomicon,

"That is not dead which can eternal lie,

And with strange aeons, even death may die."

Even in his sleep, Cthulhu's thoughts influence the dreams of his half-human worshippers, who work towards the day when Cthulhu awakes[?] and the Old Ones return. The best known chant of his worshippers (in a non-human tongue) is:

"Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn!"

"In his house at R'lyeh, dead Cthulhu lies dreaming."

At the present time, the Old Ones can be called to this universe only for brief periods, though ususally long enough to result in the death or madness of the principal character of the story. The Old Ones arouse fear in humans by their sheer wrongness--they simply do not belong to this universe--even the geometry of the buildings of R'Lyeh appears subtly non-Euclidean, and the Old Ones themselves are adorned with numerous tentacles and suckers; Cthulhu is a titanic protoplasmic mass of vaguely humanoid form, though with many cephalopod-like qualities. Although interactions with the Old Ones frequently results in death or insanity, the Old Ones are not evil. Rather they have their own purposes for which human beings are simply irrelevant.

There is also another group of these alien beings, whom the Old Ones fear, known as the Elder Gods[?], who can sometimes assist in the fight against the Old Ones, but they were more an invention of later writers, particularly August Derleth.